The present invention refers to a new process for preparing aromatic carbamates from aromatic amines and organic carbonates in the presence of a tetranuclear zinc catalyst.
Aromatic carbamates are valuable intermediates which can be used for the production of phytodrugs, dyes, pharmaceutical compounds and aromatic isocyanates used in the synthesis of polyurethanes.
Among aromatic carbamates, those of greatest interest from a commercial point of view are carbamates derived from 4,4′-methylenediphenylamine (MDA), also known as 4,4′-diaminodiphenylmethane, its isomers and/or homologues or mixtures of the aforementioned compounds as obtained by acid catalyzed condensation/rearrangement reaction of aniline and formaldehyde, as well as 2,4-toluenediamine (TDA) or technical mixtures of the two TDA isomers 2,4-TDA and 2,6-TDA (about 80/20 mixtures). The mentioned aromatic amines are used in the preparation of methylenediphenyl diisocyanate (MDI) and toluene diisocyanate (TDI). At present these isocyanates are produced industrially by phosgenation of the corresponding amines with phosgene.
In the prior art, processes are known for the production of carbamates, which are based on the functionalization of aromatic amines Ar—NH2 with organic carbonates R2CO3 in the presence of suitable catalysts, according to the following scheme:

In the case of aromatic diamines Ar(—NH2)2, biscarbamates are formed in a two step reaction, being the respective monocarbamates formed in the first step according to the following scheme:

Taking into account the alkylating properties of organic carbonates, N-alkylation competes with N-alkoxycarbonylation, and consequently N-alkylated products might be formed along the reaction.
In the U.S. Pat. No. 3,763,217 it is disclosed that Lewis acids are suitable catalysts for reacting an organic carbonate with an aromatic amine to prepare carbamates. The following catalysts are disclosed in that US patent: SbCl5, SbCl3, SbCl2, AlCl3, SbF3, FeCl3, UO2(NO3)2, UO2, UO3, NbCl5 and ThCl4.
In the U.S. Pat. No. 4,268,683 it is disclosed that zinc and divalent tin salts of monovalent organic compounds having a pKa value of at least 2.8 provide the desired carbamates in higher yields and/or selectivity than the particular Lewis acids disclosed in the U.S. Pat. No. 3,763,217. In particular the following zinc compounds are disclosed: zinc acetate, zinc acetate dihydrate, zinc naphtenate, zinc salts of fatty acids, zinc pivalate, zinc benzoate, zinc acrylate, zinc p-chlorobenzoate, zinc phenoxide, zinc oxyacetate ((AcOZn)2O), zinc chloride, zinc propionate, zinc formate, zinc chloroacetate, zinc trifluoroacetate, zinc salicylate, zinc oxalate, and zinc acetylacetonate.
In the European patent application EP-A-0065026 it is disclosed a process for preparing a carbamate from an organic carbonate and an aromatic amine in the presence of catalytic quantities of a Lewis acid catalyst, which is soluble in the reaction mixture at the reaction conditions and is a member of the group consisting of a zinc or divalent tin halide, a zinc or divalent tin salt of a monovalent organic compound which has a pKa value of at least 2.8, and a zinc or divalent tin salt of trifluoroacetic acid. Among the zinc salts are mentioned: zinc chloride, zinc acetate, zinc acetate dihydrate, zinc oxyacetate ((AcOZn)2O), zinc naphtenate, zinc octoate, zinc propionate, zinc salicylate, zinc pivalate, zinc acrylate, zinc p-chlorobenzoate, zinc phenolate, zinc formate, zinc chloroacetate, zinc acetylacetonate, zinc oxalate, and zinc trifluoroacetate.
In the article of Baba et al., Catalytic Synthesis of Dimethyltoluene-2,4-dicarbamate by the Methoxycarbonylation of 2,4-Toluenediamine with Dimethyl Carbonate Using Zn(OAc)2.2H2O, Science and Technology in Catalysis, 2002, 149, it is disclosed the reaction of the amines MDA and TDA with dimethyl carbonate in the presence of a metal salt as catalyst to obtain the corresponding dicarbamates. Several salts of zinc, tin, lead and bismuth are disclosed. It is disclosed also that the selection of the metal salt is crucial for the formation of the carbamates. Among the catalysts some zinc carboxylates showed catalytic activity and other were inactive. For example in the reaction of TDA with dimethyl carbonate and zinc acetate dihydrate as catalyst yielded 92% of dicarbamate, zinc propionate yielded 20%, whereas zinc formate was inactive.
In the article of Baba et al., Catalytic methoxycarbonylation of aromatic diamines with dimethyl carbonate to their dicarbamates using zinc acetate, Catalysis Letters, 2002, 82(3-4), 193-197, it is disclosed the preparation of dicarbamates by methoxycarbonylation of TDA and MDA with dimethyl carbonate using zinc acetate dihydrate, Zn(OAc)2.2H2O, or zinc acetate, Zn(OAc)2, as catalysts. The yield in the methoxycarbonylation of TDA with dimethyl carbonate using the hydrated catalyst is 92%, and using the non hydrated catalyst it is 98%. In the case of MDA the yield with zinc acetate dihydrate as catalyst is 98%.
In the article of Baba et al., Characteristics of methoxycarbonylation of aromatic diamine with dimethyl carbonate to dicarbamate using zinc acetate as catalyst, Green Chem., 2005, 7, 159-165, it is disclosed the reaction of aromatic amines, TDA and m-phenylenediamine, with dimethyl carbonate in the presence of zinc acetate dihydrate as catalyst.
EP-A-1268409 describes the usage of zinc acetate dihydrate as catalyst in a continuous process for the manufacturing of aromatic carbamates by reaction of 80/20 mixtures of the two TDA isomers 2,4-TDA and 2,6-TDA with dimethyl carbonate. Among other compounds, Zn salts (e.g. zinc acetate or zinc acetate dehydrate) are mentioned in EP-A-1255728 as catalysts for the synthesis of aromatic carbamates by reaction of aromatic amines like 80/20 mixtures of the two TDA isomers 2,4-TDA and 2,6-TDA with dimethyl carbonate.
Compounds or salts of especially Sn, Zn or Pb are described as catalysts for the reaction of 2,4-TDA or technical mixtures of the two TDA isomers 2,4-TDA and 2,6-TDA with diethyl carbonate in EP-A-520273 or for the reaction of MDA (that is 4,4′-MDA, its isomers and/or homologues or mixtures of the aforementioned compounds as obtained by acid catalyzed condensation/rearrangement reaction of aniline and formaldehyde) with dialkyl carbonates like dimethyl carbonate or diethyl carbonate in EP-A-510459.
In the European patent application EP-A-1958940, the inventors disclose processes for preparing azolynes, cyanoazolynes, symmetrical and unsymmetrical bisazolynes, amides, bisamides, cyanoamides, and peptides, which comprise the use of a metal catalyst defined by the general formula Zna(OCOR7)bZ2c, wherein R7 represents an optionally substituted alkyl group or an optionally substituted aryl group; Z2 represents an oxygen atom, a sulfur atom, or a selenium atom, “a” represents 1 or 4, “b” represents 2 or 6, and “c” represents 0 or 1; and provided that when “a” is 1, “b” is 2 and “c” is 0, and when “a” is 4, “b” is 6 and “c” is 1. The following zinc salts are disclosed in that patent application: zinc acetate, zinc trifluoroacetate, zinc acetoacetonate, zinc acetylacetonate, zinc trifluomethanesulfonate, and zinc p-toluenesulfonate. It is disclosed also that multinuclear zinc clusters may be used as catalysts, for example: Zn4(OAc)6O, Zn4(OCOEt)6O, Zn4(OPv)6O, Zn4[OCO(CH2)16CH3]6O, Zn4(OCOPh)6O and Zn4(OCOCF3)6O, wherein Ac represents an acetyl group, Et represents an ethyl group, Pv represents a pivaloyl group, and Ph represents a phenyl group. The zinc cluster Zn4(OAc)6O is used in the preparation of oxazolynes and peptides. However, EP-A-1958940 does not disclose or suggest the use of the aforementioned catalysts such as Zn4(OAc)6O for the preparation of aromatic carbamates.
Taking into account the economic importance of carbamates as isocyanates precursors, it is necessary to develop new and improved alternative processes for preparing carbamates in high yield and low amounts of by-products.